Yep Brendan, I use bonded V69/T70 all the time for silnylon. Its what the military specs for parachutes and what hot air-balloon companies use so I figure might as well use what they use. I have to say though, the thicker thread has alot less give so you have to make sure your bobbin tension and top tension are right. I am not sure what kind of machine you have but the only downside is if your machine has a small bobbin you will be changing it out ALOT more often. If you buy pre-wound bobbins its not a big deal but if you wind your own then get ready to be frustrated.

Lawson sez: "I think instead of calling it a Faux Flat Felled we should come up with another name as Faux implies that its a lesser quality version. This is not true as its a different type of stitch all together and is sewn in a completely different manner and in my testing I have found it to be just as strong as a true flat felled seam."

No need for a new name - it isn't faux at all. That is indeed a flat felled seam in all its glory. What people are thinking of as a genuine flat felled seam is not that; it is a lap-fold seam - which as has been pointed out is quite difficult to do without a folder attachement on the machine.

All the discussion of seam strength makes me think back over the last 40 years and I can't recall ever having a seam fail on any gear I have ever made unless it was either improperly done (bad thread tension or too small a seam allowance - been nailed by both) or due to unraveling fabric (mostly on heavier fabrics with skimpy coatings, or on uncoated fabric). So have no fear of the flat felled seam not being strong enough.

First off, a flat felled seam is different from a true lap seam - which is what is done on a double needle machine with a folder. A flat felled seam is sewn on a dingle needle machine and takes 2 (or 3 if you want) passes under the needle. A lap seam on a double needle machine is sewn in one pass - the fabric is fed into the folder side by side, the folder laps the fabric into each other ( like you hook your hands together).

You cannot do a true lapped seam without a double needle machine, so don't even try unless you want to spend the money. I use a Tex 60 (not bonded) thread on the LIghtHeart Gear tents - I recently switched to the non bonded thread as it gives a tighter seam - trust me, it's a real pain in the rear for me sewing, but on the user end, it is a much better choice. it wont' pull out of the seam if a thread should break. it also swells and fills the needle holes better than a bonded thread. From my end, I would rather use a bonded thread as it's easier to sew with, and doesn't lint up my machines, but from an end user standpoint - I want the best quality in the finished product. I also use a size 90 needle.

Yes, on a felled seam, the first pass is just through 2 layers of fabric, and those needle holes will show more than on a lapped seam because there is nothing under them to support the stitches - this came up recently here on BPL when someone had a cuben tent or tarp he just got and the holes were really showing - I pointed out that it was a felled seam, not a lapped seam -

I dont use pins ever, no iron, you make sure the seams match up with a few well placed notches, and you check several times that the bottom layer is not feeding in too much (this happens on drop feed machines) several times as you sew a long seam.

To sew on lightweight fabric such as sil or cuben, using a heavier thread such as I use and a longer stitch length, you really do need racing pullers on the machines in order to achieve a nice flat non puckered seam. Most of my machines have pullers .

Now you all have got me more confused than ever about the terminology, BUT, am still clear about the substance.

Looks like my text and diagram failed to communicate. Not the first time.

There is no argument about differing strengths. With any of these seams, there is a line of stitching, therefore a line of perforations, that are pulled on when the the fabric is tensioned along the seam. As with force 10 winds blowing at, and sometimes through, a tent, often in bursts that are positively scary. With poor fabric, the line of perforations will operate just as they do on a postage stamp. With good quality nylon or polyester, the perforation holes will elongate a little in the direction of where the tension comes, but the fabric will not fail.

With a flat felled seam, to use the term as defined by Judy here and on the companion thread, when those holes elongate, there will be four layers of fabric penetrated at each hole. And there will be sealer all through the needle holes penetrating those four layers of fabric. Even if the seam is sewn less than perfectly, there will be three layers of fabric penetrated.

But with a faux felled seam, often seen on cheaper tents, it is a different story. The diagram I placed in the above post is of a faux felled seam. Please excuse the term 'faux' if you don't like it. It is the most common way seen in the literature that's used to distinguish it from a flat felled seam. And I think 'faux' is a fair term, because it looks like a flat felled seam, but isn't. For that reason, I can't respond to Lawson's picture. To tell the difference between the real and faux, it is necessary to eyeball it very closely, or touch both sides of the seam with thumb and forefinger.

In the diagram I posted above, the one layer of fabric leaving the stitch line on the right has a row of stitch holes that has absolutely nothing under them except maybe some sealer if the article was also sealed on the inside. Nylon is a highly elastic material. Polyester no where near as much, but try and find good HH coated polyester in the one ounce range. So when the silnylon is tensioned away from the seam, those stitch holes will elongate, and the only thing in that long line of stitch holes between you and wind driven rain will be a miniscule amount of sealer.

I have indeed experienced the result of this when the sealer ages a bit - I got very wet and miserable. Sealing over the aged sealer doesn't work well either. Tried that. Maybe not quite so bad with silicone rather than PU, but still a problem. A problem not worth bothering with if it can be totally avoided with a flat felled seam that puts layers of fabric at all the stitch holes.

Hope everyone groks what a flat felled seam is - just two folded edges interlocked and sewn a mm or so inside the folds with two lines of stitching. Can do a diagram if needed.

With heavier fabrics and smaller items, all of the above may not be an issue, as has been suggested. In packs, simple french seams can be used, sealed, and the binder strip sewn over the seam. That's what Eureka and others do on tents. But there is no reason not to do it on packs in areas where an overlapping seam just isn't worth the trouble. With stuff sacks, though, a more water resistant seam may be worth the trouble. It is for me, having seen what happens once water gets into the pack after hiking in the open in the driving rain all day.

But a word of caution - the Dimension-Polyant fabrics behave more like Cuben than nylon when stitched; in that the stitch holes don't have the 'self-sealing' qualities of nylon. Sure, the material is 'waterproof' before sewn; but once lines of stitch holes are added at the seams ... . That's one of the reasons I stuck with nylon, especially when there are lighter sil-coated nylon balloon cloths and other products out there now to experiment with.

I apologize if the above sounds pendantic, which it probably does. Sorry. Maybe some good will come of it, though.

Here is a drawing I did comparing the seams. The right way to do a flat felled seam, the wrong way to do a flat felled seam, and the knock off seam. The red lines signify the stitching.

In my opinion, I think the right way to do a flat felled seam and a knock off seam with a third row of stitching are very similar in strength and I would take a piece of gear with either of them. If you have a double needle machine then you should do a true flat felled and get the seam done in one pass. If you have a single needle machine then you should do the knock off seam and for added strength its always a good idea to do a third row of stitching.

the triangle cut is called a notch. They are, as you surmised use to ensure pieces are lined up correctly, and sometimes to make it clear which is the front and what's the back (consider a one piece sleeve, it's hard to figure out without thinking hard. Notches are easier.) They solve a different problem than the proposed sharpie marks. (Also, I've never seen a gear pattern that's included any, but that's another problem.)

When I'm doing things that I really care to get lined up exactly right, I will draw two lines, one on the top piece, showing my where the first line of stitching belongs, the second on the bottom showing where the edge of the top piece should be. I don't use a sharpie, but a pencil or chalk, something that ill come out. (and which makes finer line. Even a fine tipped sharpie line is huge.) I've also got a collection of compensating feet (sadly, not generally available for home machines), which make the job easier.

I can't imagine using #69 bonded nyon on even 70d silnylon. Most of my work is with tex 30 and 40 threads and 11 and 12 (75 and 80 metric, I think) needles. I don't have a problem getting tensions set up right for those combinations.

I played around with this a bit last weekend and a lot of what I said above didn't end up being true. Adding extra layers does improve strength, as does a third stitch, although provided your stitching is straight and parallel I think the modified felled (or whatever we're calling it now) is plenty strong.

If the extra bit of fabric backing up the holes matters (not sure it does, especially if using a non adhesive sealer), here's a solution that's not really any more difficult that the regular modified felled seam, plus adds an extra layer for the first stitch:

Basically, rather than sewing the two pieces together flat, fold over the seam allowance that you normally fold under in step 2 and sew through that in step 1

Step 2 same as usual

You have a raw edge, but doesn't really matter with sil, especially after it's sealed. Folding the "flap" to the left in the above photo in step 2 hides the raw edge and gives the extra strength (which does reduce the hole elongation) but doesn't have the little bit of fabric covering the hole.

Brendan's way looks very promising as well and as you can see its a nice looking seam. I will have to give this way a try.

I completely agree that adding extra layers increases the strength. Its the reason parachute and hot air balloon manufacturers add tape to the bottom side of seams. Extra reinforcement = extra strength.

Good eye, it's actually a 401a. I have two industrial machines and that singer's still my favorite to use, especially for light fabrics. I found it at a thrift store, cleaned it up to sell, and that was three years ago. Don't think it's going anywhere.

Ivo,I've found with silnylon that bonding the seam takes away much of the elasticity of the fabric, but just at the seam. This prevented the tent canopy from becoming taut.When the seam was redone without adhesive, just thread at tension low enough to allow some stretch, the canopy became fully taut when tightened over the pole frame.

But I was using undiluted GE silicone glue. Perhaps your diluted mixture will yield a different result.

Lawson, Thank you for those excellent seam diagrams. They really cleared up for me what you were saying.

I really wonder if the second, or wrong way diagram is really all that wrong. The reason being that a stitch line on the right way diagram that is less than a mm from the raw edge of material may not contribute all that much to the seam's overall strength. If one just sewed an edge to another piece of fabric with a stitch line no more than a mm from the raw edge, it would be very easy to rip the edge off, as the thread would just pull out through the one mm of material. I don't think the elastic silcoat on silnylon is going to retard that very much from happening. On the other hand, if sewing the seam the right way is going to add significantly to strength, I don't mind taking the extra care.

Your third diagram, the knock-off, appears to be a modified, or rolled seam along the lines that the OP had in mind. Without knowing exactly how strong it is, i agree that it is plenty strong for its purpose. The only issue I have with it is stitch holes opening up at one end of the seam, and thereby compromising the water resistance of the canopy material.

Your first, or right way diagram, appears to be the flat felled seam that is frequently used on high quality tents and tarps. Hope that is correct.

Brendan,Without being sure that my understanding of the seam you pictured is correct, it appears to be a very savvy way of getting the benefits of a flat felled seam without a double needle machine, or having to use pins. As best I understand, it does add a few extra layers of fabric to the seam, but the weight added may be negligible. One reason for pins, is to help insure that the fabric on one side of the seam doesn't stretch more than the fabric on the other, so that the fabric panels at the end of the seam don't line up as intended.

One dilemma for me is that when there is no pattern, and I am tailoring the netting door and vestibule to the front edge of the canopy with pins, the rolled seam seems to be the only approach that comes to mind. This does allow the stitch holes at the front of the canopy/vestible seam to elongate and potentially leak. I'm still trying to come to grips with that one.

Actually, the reason there are five layers is because the seam allowances are the same for the first seam; the two edges of the fabric are aligned, then the seam is sewn with a large seam allowance, then the seam is flattened out, and the seam allowances are folded in half and tucked under itself. Then you sew two lines of stitching through the five layers.

It looks like the seam allowances are different because the diagram is a much-expanded view; in reality the 'vertical' distances are minimal.

You can use double and single seam allowances to make a double-lapped seam, in which case the single allowance is on the bottom of the stack as shown, and the double allowance is folded under it.